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Optics & Laser Technology 40 (2008) 113119

Impact on textile properties of polyester with laser

C.W. Kan

Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, China

Received 14 November 2006; received in revised form 2 March 2007; accepted 16 March 2007

Available online 30 April 2007

Abstract

Modification of the textile properties of polyester due to laser irradiation were studied and these properties included fibre weight and

diameter, tensile strength and elongation, yarn abrasion, bending, surface lustre, wetting, air permeability as well as crystallinity.Properties such as wettability and air permeability were positively affected while properties of fibre weight and diameter, tensile strength,

yarn abrasion and bending were adversely affected. In this study, laser irradiation used was not found to affect the bulk properties of

polymer due to its low penetration depth, and hence, the effect of laser irradiation on bulk and structural properties was limited.

However, the performance and comfort properties of the laser-irradiated polyester could be significantly affected by laser irradiation.

r 2007 Elsevier Ltd. All rights reserved.

Keywords: Polyester; Laser; Textile

1. Introduction

In the last decade, considerable effort has been made in

developing surface treatments such as UV irradiation,plasma, electron beam and ion beam to modify the

properties of textile materials. Laser modification on

material surface is one of the most studied technologies.

It has been shown that materials like polymers, woods,

metals, semiconductors, dielectrics and quartz modified by

laser irradiation often exhibit physical and chemical

changes in the materials surface. In the case of polymers,

some well-oriented structure of grooves or ripple structures

with dimensions in the range of micrometre are developed

on surface with irradiation fluence above the so-called

ablation threshold, e.g. a polyester irradiated by a laser of

wavelength 248 nm with energy of about 30 mJ/cm2 [1,2].

The laser irradiation of highly absorbing polymers such as

polyester can generate characteristic modifications of the

surface morphology [3,4]. The physical and chemical

properties are also affected after laser irradiation [57].

Hence, it is reasonable to believe that such surface

modification of a polymer may have an important impact

on its textile properties [810]. In order to find out which

property of polyester is affected by laser irradiation,

different experiments and testing instruments were used

to characterize the treated samples.

In this paper, the influence of laser irradiation on someof the important textile properties of polyester was studied.

These properties include fibre weight and diameter, tensile

strength and elongation, yarn abrasion, bending, surface

lustre, wetting, air permeability and crystallinity.

2. Experimental

All measurements were undertaken with 5% tolerance

limit.

2.1. Material

Commercially available poly(ethylene terephthalate)

(polyester) fibre, yarn and fabric were used for the study.

All samples were conditioned for at least 24 h prior to

experiment under the standard conditions at 20 1C and

65% relative humidity.

2.2. Laser irradiation

Both high- and low-fluence laser irradiations were con-

ducted [810]. Irradiation was performed using a commercial

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pulsed UV excimer laser (Lambda Physik COMP EX 205)

under atmospheric conditions in air. The laser was operated

with KrF gas and a wavelength of 248 nm was produced. In

high-fluence laser irradiation, samples were irradiated directly

from the laser beam without using either special photomask

or focusing lens. The laser energies in terms of applied fluence

and number of pulses varied in different experiments.The laser fluence was regulated in the range from 50 to

150 mJ/cm2 and the number of pulses varied between 0 and

200. In low-fIuence laser irradiation, only the highly

polarization beam method was employed and the fluence

was controlled to 2000 pulses at 6 mJ/cm2. For both types of

laser irradiation, the pulse repetition was kept constant at

1 Hz to avoid any possible heat accumulation.

2.3. Morphological study

The morphology of samples was investigated by Scan-

ning Electron Microscopy (SEM) (Lecia Stereoscan 440).

2.4. Bulk and structural properties

The degree of crystallinity was determined by integrating

the peak area of a differential scanning calorimetry curve.

This method is based on a thermodynamic definition of

order and it requires the absolute value of the ideal

polymer crystals heat of fusion (DH of melting peak of

ideal polyester crystal 119.8 J/g) [11]. The heat of fusion

obtained from this is directly proportional to the crystal-

linity. The measurement was conducted by using a Mettler

DSC Model 25 under thermally controlled conditions

with a heat rate of 10 1C/min in a nitrogen atmospherewith the scanning ranging from 30 to 300 1C. The degree of

crystallinity was calculated based on the following equa-

tion [11]:

degree of crystallinity

DH of melting peak

DH of melting peak of ideal polyester crystal

100%.

2.5. Weight and fibre diameter change

These experiments were designed to compare the sample

weight and fibre diameter before and after laser irradiation;

the percentage change of the sample weight and fibre

diameter could then be obtained. The fibre diameter was

obtained directly from microscopic measurement.

2.6. Performance properties

The tensile strength and elongation-at-break of the fibres

were measured in accordance with ASTM D2101-2005

using an Instron Tensile Tester 4466. The yarn abrasion of

the fibres was examined by a Shirley Yarn Abrasion Tester

with a silicon carbide waterproof abrasive paper 100

(etectro-coated) for the creation of abrasion against the

specimens. KES-F (Kawabata Evaluation System for

Fabrics) Bending Tester was employed for measuring both

the bending rigidity, B, and hysteresis, 2HB, of the yarn

samples. Specimen each comprised of 82 ends was tested.

The glossiness of the fabric samples was measured by aGoniophotometer (Model VG-ID, Nippon Denshoku

Kogyo Co. Ltd.) designed according to Japanese Industrial

Standard 28741. During the experiment, the incident angle

of light was always fixed at 601 while the receiving angle

varied from 01 to 851 at the interval of 51. This

measurement method is widely used since it facilitates

subjective evaluation in which the incident light and the

fabric sample are both fixed while the position of the

observer is varied. The glossiness index expressed in

terms of percentage was measured in both warp and weft

directions.

2.7. Comfort properties

The vertical drop test of fabrics was measured in

accordance with BS 4554. A drop of distilled water was

placed onto the fabric sample and the time, Ts, for which

the liquid required to sink into the fabric completely (i.e.

the liquid is no longer visible from the surface of the fabric)

was recorded. The shorter the time, the more wettable was

the fabric. A Shirley air permeability tester was employed

to measure the air permeability of the fabric samples based

on BS 5636 and the air permeability in this test is defined as

the volume of air in milliliters passed in 1 s through

100smm2

of the fabric at a pressure difference of 10 mmhead of water. Specimens with size 15cm2, avoiding

selvages, crease and fold, were used. The equation shown

below was used to calculate the air permeability of the

samples.

Air permeability Air flow

5:08.

3. Results and discussion

3.1. Degree of crystallinity

Table 1 shows the degree of crystallinity of the laser-

irradiated polyester. The change of crystallinity of polye-

ster as a result of laser irradiation was 2% for high fluence;

no change was observed under low-fluence irradiation. It

seems that the overall change in the degree of crystallinity

is insignificant. The relatively small difference in the degree

of crystallinity between the laser-irradiated and untreated

polyester is expected and explainable, although it is

reasonably believed that the surface of the high-fluence-

irradiated polyester would become highly amorphous due

to ablation. The small change may be due to the nature of

laser irradiation as it generates only surface morphological

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modification where the bulk properties of polymer remain

unchanged. In fact, the penetration depth of laser energy is

limited to a few micrometres under high-fluence irradiation

and even less for the low-fluence counterpart, while

crystallinity is a bulk internal property of polymer.

3.2. Morphological study

Fig. 1 shows the surface structure of the untreated

polyester fibre and a smooth surface is noted. With the

influence of laser irradiation under high fluence, certain

roughness or periodic roll was developed on the polyester

fibre surface as shown in Fig. 2, which is called roll-like or

ripple-like structure. The orientation of this kind of ripple-

like structure is strictly perpendicular to the fibrillar

orientation of the fibre. On the other hand, periodic

surface structure of sub-micrometre size was developed

when the polyester surface was irradiated with low fluence

as shown in Fig. 3. In Fig. 3, the polyester has sub-

micronmetre ($

200 nm) ripple spacing on the surface andsome with granule structure on the two sides after laser

irradiation. The granules are distributed quite randomly

which may be due to the geometry effect of the curved

surface.

3.3. Weight and fibre diameter change

The weight change of polyester caused by high-fluence

laser irradiation is shown in Fig. 4. It can be observed that

the weight change correlated well with laser fluence and the

number of pulses applied during laser irradiation, i.e. an

increasing fluence and number of pulses resulted in an

increasing level of weight loss. In the first few pulses, the

weight loss was insignificant but increasing the pulse counts

would further increase the weight loss. However, no

saturation of weight loss was seen against the pulse counts.

The reduction in weight with increased laser irradiation

may be due to the etching effect induced by the high-

fluence ablation as the polyester fibre was etched pulse-by-

pulse during fluence above the ablation threshold [12]. In

the case of high-fluence irradiation, materials were etched

away and ejected from the surface into the atmosphere

during ablation. On the other hand, no significant weight

change was recorded for the low-fluence irradiation. The

energy involved in low-fluence treatment was limited to a

very low level and hence no obvious ablation was resulted

and no material would be removed or etched from the

polyester surface significantly under such irradiation [13].

Fibre diameter measurement is shown in Fig. 5 for high-

fluence-irradiated samples only as no significant change in

fibre diameter was recorded for low-fluence counterparts.

In the high-fluence irradiation, an increase in the number

of pulses was resulted with an increase in diameter

reduction. For the first 10 pulses, the reduction was almost

insignificant but once the pulse count went beyond 20, the

reduction became more severe.

3.4. Tensile strength and elongation

The results of the breaking load and elongation-at-

break of the polyester samples with and without laser

irradiation are listed in Table 2. Under the high-fluence

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Table 1

Degree of crystallinity of laser irradiated polyester: high fluence 5 pulses

at 70 mJ/cm2 and low fluence 2000 pulses at 6 mJ/cm2

Degree of crystallinity

(%)

% Changes over

control

Control 44

High-fluence

irradiated

43 k2

Low-fluence

irradiated

44

Fig. 1. Surface structure of untreated polyester fibre.

Fig. 2. Surface structure of polyester fibre under high fluence (5 pulses at

100 mJ/cm2).

C.W. Kan / Optics & Laser Technology 40 (2008) 113119 115

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laser irradiation, irradiation could reduce both the break-

ing load and elongation of the sample. The rate of

reduction for both properties was similar in terms of

percentage, which was about 10%. The reduction could be

due to the fact that the induced high-fluence ripples on the

surface, as shown in Fig. 2, created more weak points to

the fibre and eventually reduced both strength and

extensibility. Another reason may be due to the deposition

of debris as the debris could give a harder surface to

polyester, making it more rigid resulting in lower strength

and elongation.

After low-fluence irradiation, no significant change in

tensile strength and elongation was observed. The small

reduction in the breaking load and elongation of the

irradiated sample has little meaning statistically. It may beargued that the ripples induced by irradiation will result in

more weak points to the fibre; hence the tensile strength

should be reduced [8]. However, bearing in mind that the

depth of ripples is in the range of the sub-micrometre level,

as shown in Fig. 3, the weakening effect to the strength and

elongation should be very little.

3.5. Yarn abrasion

Table 3 shows the yarn abrasion results; a reduction of

approximately 13% and 10% for high- and low-fluence-

irradiated polyester yarn was recorded, respectively. Unlike

previous results, no obvious difference was noted between

both the high and low fluence of laser irradiation. The yarn

abrasion tester employs the principle of using a rotary rod

wrapped with abrasive paper creating abrasion against the

yarn specimen. Hence, laser-irradiated yarn would ob-

viously have a lower abrasion resistance since it is full of

periodical ripple-like structures and these structures would

largely reduce the contact areas between the abrasive paper

and the yarn [9]. Also, these laser-developed structures

induced more weak points to the yarn sample and therefore

when the yarn was under tension, the fibre arrangement

would be changed resulting in lower yarn abrasion

resistance.

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0

5

10

15

20

25

0 50 100 150 200

No. of laser pulses

Reductioninweight(%)

Fig. 4. Weight reduction of polyester after laser irradiation (~: 50 mJ/cm2,: 150 mJ/cm2).

0

5

10

15

20

25

0 50 100 150 200 250 300

No. of laser pulses

Diameterreduction(%)

Fig. 5. Fibre diameter change after laser irradiation: at 50 mJ/cm2.

Table 2

The breaking load and elongation-at-break of laser-irradiated polyester:

high fluence 10 pulses at 50 mJ/cm2 and low fluence 2000 pulses at

6 mJ/cm2

Breaking load (N) Percentage of elongation-

at-break (%)

Control 0.106 11.67

High-fluence

irradiated

0.095(k10%) 10.44(k11%)

Low-fluence

irradiated

0.103(k3%) 11.30(k3%)

The percentages in the parentheses indicate the increase/decrease in the

value of irradiated samples compared with the control.

Table 3

Yarn abrasion of laser-irradiated polyester: high fluence 10 pulses at

50mJ/cm2 and low fluence 2000 pulses at 6 mJ/cm2

Mean revolutions % Changes over control

Control 68

High-fluence irradiated 59 k13

Low-fluence irradiated 61 k10

Fig. 3. Surface structure of polyester fibre under low fluence (2000 pulses

at 6 mJ/cm2).


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3.6. Bending property

The bending properties have important effects on both

the handle and tailoring performance of textile materials.

Table 4 shows the bending properties of the laser-irradiated

polyester yarn, including bending rigidity and bending

hysteresis. The results in Table 4 indicated that the bendingrigidity and bending hysteresis increased for both types of

laser irradiations. A higher increment was observed for

high-fluence-irradiated samples as the bending rigidity and

bending hysteresis increased by 20% and 50% respectively.

Under low-fluence irradiation, the increment was relatively

small. The bending rigidity and bending hysteresis in-

creased by 7% and 25% respectively.

The increase in the bending properties may result in

difficulties in manufacturing, especially for the dramatic

increase in bending hysteresis. Generally speaking, the

smaller the values of the bending hysteresis, the better will

be the fabric bending recovery ability. The tremendous

increase in the bending properties of the laser-irradiated

yarn may finally decrease the fabric flexibility and elastic

recovery from bending action [14], which may in turn affect

the fabric tailorability, draping quality and wear. Highly

skill operators may therefore be needed to deal with the

problem and thus the cost of manufacturing may increase.

Although the fibre diameter was reduced after laser

irradiation, the ripples were formed on the polyester fibre

surface. Thus, during the bending process, the ripples so

formed on the polyester fibre surface may increase the

friction between the fibre in the yarn which restricts the

bending moment of yarn and hence the bending property

changed.

3.7. Surface lustre

Surface lustre is the term generally accepted to describe

the glossiness of a textile material and is defined as the

intensity of directionally reflected light from a surface. The

glossiness indexes of the polyester fabrics before and

after laser irradiations were measured in both warp and

weft directions as shown in Figs. 6 and 7 respectively.

The glossiness distribution curves in warp and weft

directions produced similar trends for the sample tested.

No prominent peak value appeared in either directionwhere the glossiness in warp was slightly higher than that

of the weft.

Fig. 2 illustrates the effect of high-fluence laser irradia-

tion on the polyester fibre surface. It was observed that the

ripples structure was formed perpendicularly to the fibre

axis with the mean roll distance of approximately 1mm.

While in the low-fluence laser treatment, ripples were also

formed but the effect was not as significant as in the high-

fluence treatment. High-fluence-irradiated sample would

have a lower level of glossiness index, whereas low-fluence-

irradiated counterparts would increase the overall glossi-

ness index. Laser irradiation is claimed to largely lower the

glossiness index of textile material with sharp peaks thus

making the material more silk-like [15] because the surface

of laser-irradiated area would diffuse the incident light

more evenly and eventually reduce the specular reflection.

However, it is not known that why there would be an

overall increase in the glossiness index for the low-fluence-

irradiated sample. At the present stage, the most possible

explanation would be because the process of low-fluence

irradiation could be able to clean up the surface of the

irradiated part such as removing the dust particles and

minimizing the manufacturing defect by smoothing the

surface. Such effects would eventually increase the glossi-

ness index.

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Table 4

The bending properties of laser-irradiated polyester: high fluence 10

pulses at 50 mJ/cm2 and low fluence 2000 pulses at 6 mJ/cm2

Bending rigidity, B

(gfcm2/cm)

Bending Hysteresis, 2HB

(gfcm/cm)

Control 0.0015 0.0004

High-fluence

irradiated

0.0018 (m20%) 0.0006 (m50%)

Low-fluence

irradiated

0.0016 (m7%) 0.0005 (m25%)

The percentages in the parentheses indicate the increase/decrease in the

value of irradiated samples compared with the control.

0

0.51

1.5

2

2.5

3

3.5

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Recevier angle

Glossiness

Fig. 6. The glossiness distribution curve (warp direction) of laser-

irradiated polyester. n Control, } 5 pulses at 100 mJ/cm2 and

o 2000 pulses at 6 mJ/cm2.

0

0.5

1

1.5

22.5

3

3.5

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85

Recevier angle

Glossiness

Fig. 7. The glossiness distribution curve (weft direction) of laser-

irradiated polyester. n Control, } 5 pulses at 100 mJ/cm2 and

o 2000 pulses at 6 mJ/cm2.


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3.8. Wetting

Table 5 shows the vertical drop test result for both high-

and low-fluence-irradiated samples. It was observed that

high-fluence laser irradiation increased greatly the wetting

time of the polyester fabric from 470 to 4900 s (exceeding

900 s). The large increment in the wetting time was believedto be closely related to the ripple-like structure. The ripple

structure increased the roughness of the surface and this

kind of increased roughness enhanced the unwettability of

the fabric, resulting in an increase in the wetting time.

Researchers have experimentally proved that surface

roughness decreases the wettability of a hydrophobic

material [15]. Many models have been proposed in

explaining this phenomenon. Besides, if the surface is

sufficiently rough, a liquid with a large contact angle may

not completely wet the surface. This incompletely wetted

surface provides room for air to be trapped between the

liquid and solid interface (Fig. 8), resulting in a composite

interface. This composite interface prevents water penetra-

tion and this mechanism has already been adopted by some

Japanese manufacturers in producing high-tech water

repellent textile materials.

In addition, under high-fluence laser irradiation, the

surface of aromatic polymer-like polyester will normally

result in the deposition of some yellow to black materials

often known as debris. This debris was ionized and carbon-

rich materials finally condensed to form high aggregates

[16,17], resulting in a more hydrophobic surface. Such

reduced surface oxygen content may also be part of the

reason for the increase in the wetting time.

A reduced wetting time was observed for the low-fluence-irradiated sample, which is completely opposite to

the high-fluence result. It means that the wettability of low-

fluence-irradiated sample has increased (hydrophilicity

increased). The ripples induced by low fluence were in the

range of nanometer level and it is reasonable to believe that

air could not be able to be trapped inside the gaps between

ripples due to size constraints. Hence, the effect of a

composite interface could not take place. The increased

surface oxygen content and the reduced contact angle

measurement could help reduce the time for the droplets of

water to sink into the sample. This is because the polar

radicals of the low-fluence-irradiated polyester have

increased. This increased polarity will lead to the increase

in the attraction force between the modified surface and the

polar water molecules.

4. Air permeability

Air permeability is one of the major properties of textile

materials and is governed by factors like the fabric

structure, density, thickness and surface characteristics.

Table 6 indicates that both laser irradiations could increase

the air permeability of the fabric samples. It is believed that

the contributing factor is the surface modification of the

fibres [10]. The laser irradiation induced ripple structures

on the surface of the fibres, which eventual1y providedmore air space between fibres and fabric and therefore it is

possible for more air to pass through the fabric resulting in

better air permeabi1ity. No obvious difference between

high- and low-fluence irradiations was noted.

5. Conclusions

In this paper, some common textiles properties of

polyester were studied after the high- and low-fluence laser

surface morphological treatment. Some properties were

affected significantly while the others were found un-

changed. In fact, depending on the end-uses of the

materials, for example, increased wettability of polyester

may be good for ordinary textile functions but bad for

water resistant functional garments. It is therefore sug-

gested that the correct selection of surface treatment

parameters is of prime importance. In general, laser

irradiation could not affect the bulk properties of a

polymer due to its low penetration depth, and hence, the

effect of the treatment on the bulk and structural properties

are limited. However, the performance and comfort

properties of the irradiated polyester could be largely

affected by laser irradiation as these properties could have

been changed considerably if the surface is modified.

ARTICLE IN PRESS

Table 5

Results of vertical drop test of laser-irradiated polyester: high fluence 10

pulses at 50 mJ/cm2 and low fluence 2000 pulses at 6 mJ/cm2

Seconds % Changes over control

Control 470 s

High-fluence irradiated 4 900 s ImmeasurableLow-fluence irradiated 330 s 30

Fig. 8. The effect of surface roughness on wetting.

Table 6

Air permeability of laser-irradiated polyester: high fluence 10 pulses at

50mJ/cm2 and low fluence 2000 pulses at 6 mJ/cm2

ml/(cm2s) % Ch ange s ove r co ntro l

Control 19.6

High-fluence irradiated 20.9 m 6.6

Low-fluence irradiated 21.1 m 7.7


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